Drain sharing split LNA

ABSTRACT

A receiver front end ( 300 ) having low noise amplifiers (LNAs) is disclosed herein. A cascode having a “common source” configured input FET and a “common gate” configured output FET can be turned on or off using the gate of the output FET. A first switch ( 235 ) is provided that allows a connection to be either established or broken between the source terminal of the input FET of each LNA. A drain switch ( 260 ) is provided between the drain terminals of input FETs to place the input FETs in parallel. This increases the g m  of the input stage of the amplifier, thus improving the noise figure of the amplifier.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. patent application Ser.No. 15/887,816 filed on Feb. 2, 2018, entitled “Drain Sharing SplitLNA”, the disclosure of which is incorporated herein by reference in itsentirety.

BACKGROUND (1) Technical Field

Various embodiments described herein relate to amplifiers and moreparticularly to low noise amplifiers for use in communicationsequipment.

(2) Background

The front end of a communications receiver typically includes a lownoise amplifier (“LNA”) that is responsible for providing the firststage amplification to a signal received within the communicationsreceiver. The operational specifications of the LNA are very importantto the overall quality of the communications receiver. Any noise ordistortion introduced by the LNA will cause degradation of the overallreceiver performance. Accordingly, the sensitivity of a receiver is, inlarge part, determined by the quality of the front end and inparticular, by the quality of the LNA.

In some cases, the LNA is required to operate over a relatively broadfrequency band and to amplify signals having several distinct modulatedbaseband or modulated intermediate frequency (IF) signals. One exampleof a situation in which the LNA is required to amplify a received signalhaving multiple modulated IF or baseband signals is the case in which anintraband noncontiguous carrier aggregation (CA) signal is to bereceived. A CA signal can have two channels (or IF carriers) havingfrequencies that are not adjacent to one another, but which lie withinthe frequency range that can be addressed by a single LNA amplifier. Forexample, a CA signal may have two non-adjacent channels within acellular frequency band defined by 3rd Generation Partnership Project(3GPP). 3GPP is a well-known industry standard setting organization.

In the case in which a receiver is required to receive a CA signal, suchas a cellular telephone that is compliant with the Release 11 of the3GPP communications industry standard, the LNA typically amplifies thereceived signal and provides the amplified output signal to a passivesplitter. FIG. 1 is an illustration of a portion of a cellular telephonefront end in which an LNA 101 is coupled to a variable attenuator 103. Abypass switch 105 allows the variable attenuator to be optionallyshunted. The signal is then coupled to a single pole, three throw modeselector switch 107 that allows the output of the LNA 101 to beselectively coupled to only a first downconverter and baseband circuitry(DBC) 109, a second DBC 111 or both the first and the second DBC 109,111.

When the mode selector switch 107 is in the first position (i.e., SingleChannel mode 1), the output of the LNA 101 is coupled directly to thefirst DBC 109. In the second position (i.e., Split mode), the output ofthe LNA 101 is coupled through a passive power splitter 113 to both thefirst and second DBC 109, 111. In the third position (i.e., SingleChannel mode 2), the output of the LNA 101 is coupled to only the secondDBC 111.

There are several limitations that arise from the architecture shown inFIG. 1. The first limitation is the amount of isolation that can beachieved between the first and second DBC 109, 111. Typically, awell-manufactured 3 dB splitter can achieve approximately 18-20 dB ofisolation between outputs at the center frequency for which the splitter113 is designed to operate. Signals that are cross-coupled from one DBCto the other will typically result in interference and distortion thatwill result in an overall reduction in sensitivity of the receiver.

Furthermore, passive splitters typically are designed to operateoptimally in a relatively narrow frequency range. That is, passivesplitters, by their nature are narrow band devices. As the frequency ofthe signal coupled through the splitter 113 deviates from the optimalfrequency for which the splitter was designed, the output-to-outputisolation will degrade. Due to the limitations of the splitterscurrently available, and because receivers that are designed to handleCA signals must operate in a relatively broad frequency range, thedesired isolation between the DBCs 109, 111 is difficult to achieve.

Furthermore, power splitters such as the splitter 113 shown in FIG. 1,have significant loss. Since 3 dB power splitters split the power inhalf, even an ideal splitter will result in a 3 dB reduction in powerpresented to the DBCs 109, 111 in the Split mode compared to the SingleChannel modes. In addition, most splitters will have an additional 1.0to 1.5 dB of insertion loss. The insertion loss, like theoutput-to-output isolation, will typically get worse as the frequency ofthe signals applied deviates from the center frequency for which thesplitter was designed to operate.

Still further, the losses encountered in the mode selection switch 107and the splitter 113 lead to a need for more gain. This results inreductions in linearity (as typically characterized by measuring the“third order intercept”) and degradation of the noise figure of thereceiver when operating in Split mode.

Therefore, there is currently a need for a CA capable receiver front endthat can operate in Split mode with high output-to-output isolation,without degraded third order intercept and noise figure, and withrelatively low front end losses.

SUMMARY OF THE INVENTION

A receiver front end capable of receiving and processing intrabandnon-contiguous carrier aggregate (CA) signals using multiple low noiseamplifiers (LNAs) is disclosed herein. In accordance with someembodiments of the disclosed method and apparatus, each of a pluralityof amplifiers is an LNA configured as a cascode (i.e., a two-stageamplifier having two transistors, the first configured as a “commonsource” input transistor, e.g., input field effect transistor (FET), andthe second configured in a “common gate” configuration as an outputtransistor, (e.g. output FET). In other embodiments, the LNA may haveadditional transistors (i.e., more than two stages and/or stackedtransistors). The output transistor of each LNA can be turned on or off(i.e., by either using the gate of the output FET or opening a switch inthe source of the input FET 254, 252). The gates of an input FET arecoupled together to form a common input. However, in some embodiments,the gates of the two FETs can be separated to allow the gate of an inputFET of an LNA for which the output transistor is turned off (i.e., FETsare not conducting current from drain terminal to source terminal) to beindependently controlled to turn off the input FET. A first switch isprovided that allows a connection to be either established or brokenbetween the source terminal of the input FET of each LNA. In addition, asecond switch allows a switchable gate-to-source and/or gate to groundcapacitor to be selectively applied to the input FET of at least one ofthe LNAs. In some embodiments, an additional switch is provided thatallows a source to ground degeneration inductor to be disconnected fromthe source terminal of an input FET of an LNA for which the output FETis turned off. Selectively turning the output FETs on and off allows theamplifier to operate in both a single mode and a split mode.Furthermore, use of the switches ensures that the input impedance to theamplifier is the same in single mode and in split mode. Still further, adrain switch is provided that couples the drain terminals of each inputFET together during single mode. The switch is opened to decouple thedrain terminals during split mode.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a portion of a cellular telephone front endin which an LNA is coupled to a variable attenuator.

FIG. 2 is an illustration of a front end amplifier having degenerationswitches, gate capacitor modules and using multiple LNAs operating ineither single mode or split mode.

FIG. 3 is a simplified schematic of another embodiment of an amplifierhaving source split LNAs and a drain switch and including a resistancethat can selectively be placed in parallel across an inductor andcapacitor in each output load matching circuit.

FIG. 4 is a graph illustrating the improvement in noise figure attainedby using the drain switch.

FIG. 5 is a graph and chart showing the relationship between the currentI_(DD) and noise figure as a function of input frequency.

FIG. 6 illustrates a method in accordance with one embodiment foramplifying a signal (e.g., a CA signal) using more than one amplifier.

FIG. 7 is an illustration of another embodiment of a method thatincludes removing parallel drain resistances during single mode.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 is an illustration of a front end amplifier 200 of acommunications receiver in which multiple low noise amplifiers (LNAs)202, 204 are used to amplify signals. Signals to be amplified arecoupled through a front end signal input terminal 206. In a first mode,referred to as “single mode”, one of the output FETs 208, 212 associatedwith the LNAs 202, 204 is turned on (i.e., is actively amplifying asignal applied to the input of the LNA 202, 204). The amplified outputof the active LNA 202, 204 is coupled to an output terminal 232, 234.The output FET 208, 212 of the other LNA 204, 202 is turned off (i.e.,not allowing current to flow from drain to source). In one embodiment ofthe disclosed method and apparatus, each LNA 202, 204 comprises a pairof field effect transistors (FETs) 208, 210, and 212, 214. Each pairforms a two-stage LNA in a cascode architecture. However, it will beunderstood by those skilled in the art that other types of transistorsmay be used, including, but not limited to, bipolar junctiontransistors. Furthermore, any type of FET may be used to implement theLNA, including, but not limited to metal-oxide semiconductors (MOSFETs),junction field effect transistors (JFETs), insulated gate FETs (IGFETs),metal semiconductor FETs (MESFETs), etc. While some types of transistorsmay be better suited to particular applications, the concepts associatedwith the disclosed method and apparatus do not exclude the use of anyparticular type of transistor. Still further, additional transistors canbe included within an LNA (e.g., LNA 202, 204), either as additionalamplifier stages or stacked with those FETs 208, 212 that are shown. Inaddition, in some embodiments, the particular type of transistor and thenumber of such transistors can differ from one LNA 202, 204 to anotheror within each LNA 202, 204.

LNA control signals applied to control input terminals 216, 218 coupledto the gates of the output transistors (e.g., FETs) of the LNAimplemented by the FETs 208, 212 control whether the output FETs 208,212 of each LNA 202, 204 is on or off (i.e., conducting significantcurrent from drain terminal to source terminal). In one embodiment, theLNA control signals are generated by a control module, such as an LNAController 217. The LNA Controller 217 may generate the LNA controlsignals based on information regarding the types of signals that will bereceived by the amplifier 200, the content carried by the signals, orbased on user commands to select one or more channels. The LNAcontroller 217 may be a general purpose processor capable of receivingcommands and processing the commands to generate control signals to theLNAs and associated switches disclosed throughout this disclosure.Alternatively, the LNA controller 217 is a dedicated processor speciallydesigned for generating the control signals. Those skilled in the artwill understand how to make such a processor for receiving a command toenter a first mode, such as split mode, and determine the particularconfiguration of switches and LNA control signals to be generated. Insome cases, the LNA controller 217 may be as simple as a logic blockwith look-up table. Alternatively, in some embodiments, the LNAcontroller 217 may also rely upon additional information in determiningthe states of switch control and LNA control signals.

In single mode, the LNA control signal to one of the LNAs 202, 204causes the output FET 208, 212 of that LNA 202, 204 to be turned on. TheLNA control to the other LNA 204, 202 causes the output FET 212, 208 ofthat LNA 202, 204 to be turned off. In split mode, the output FETs 208,212 of both LNAs 202, 204 are on. It will be understood by those skilledin the art that additional LNAs not shown in FIG. 2 could be coupledsimilarly to extend the amplifier to select additional channels usingadditional modes of operation.

Output load matching circuits 220, 222 coupled to the output ports 224,226 of each LNA 202, 204 provide a means by which the output impedancecan be matched to a load. In one case, an input matching circuit 228 isprovided to match the input impedance of the amplifier to the sourceterminal. The input matching circuit 228 includes an input matchinginductor 229 and an input DC block capacitor 233. In some embodiments,output shunt capacitors 230, 231 provide a relatively low capacitivereactance to ground for signals in the frequency range of the inputsignals applied to the input of the LNAs 202, 204, respectively. In someembodiments, separate V_(DD) supply voltage sources can be provided foreach LNA in order to increase the isolation between the LNAs 202, 204.In other embodiments, the same supply terminal can be used to provideV_(DD) to two or more of the LNAs.

The isolation achieved by the front end 200 shown in FIG. 2 is enhancedby the fact that there is significant isolation between the output port232 of the first LNA 202 and output port 234 of the second LNA 204. Inthe amplifier 200, the isolation between the outputs of the front endwill improve for signals that are separated by several interveningchannels. That is, as the separation in frequency increases, the levelof gain-versus-frequency overlap of one narrow band-tuned output to theother will decrease. This decrease will enhance the isolation betweenthe outputs. In lower gain modes of operation, the output isolation willimprove.

One problem that needs to be addressed when using two LNAs in thismanner is that the input impedance of the front end amplifier 200 willvary depending upon the mode in which the receiver is operated. That is,the input impedance presented in single mode will be significantlydifferent from the impedance presented in split mode largely due to adifference in gate-to-source capacitance, Cgs, of the FET transistorwhen the output FET of the LNA is on and when the output FET of the LNAis off. A large difference in input impedances will cause a large inputmismatch, which in turn creates large detrimental effects on virtuallyevery aspect of the amplifier 200. The affect can result in an increasein noise figure, a reduction in gain, and a degradation in linearity as,for example, measured by third order intercept (IP3). The relativelylarge changes in the C_(gs) of the input FET 210, 214 of each LNA 202,204 from the conducting state to the non-conducting state result inlarge changes in both the real and imaginary parts of the inputimpedance of the amplifier 200 when operating in single mode versussplit mode. A source switch 235 that can be closed in single mode tocouple the source of the first input FET 210 to the source of the secondinput FET 214 is provided to reduce this affect. Closing the switch 235to connect the sources of the two input FETs 210, 214 during single modemakes the input impedance presented in split mode (i.e., when the outputFETs 208, 212 of both LNAs 202, 204 are turned on) much closer to theinput impedance presented during single mode with the switch 235 open.However, this still represents a large impedance change as compared tosplit mode. In split mode, the source switch 235 is opened. Opening thesource switch 235 during split mode improves the noise isolation betweenthe outputs 216, 218.

In addition to the source switch 235, in some embodiments, the front endamplifier 200 has at least one gate capacitance module 240 comprising agate capacitor 242 and a gate switch 244 connected in series between afirst and second terminal of the module 240. The gate switch 244 can beswitched to insert the gate capacitor 242 in parallel with the gate andsource of the input FET 210 to provide additional input capacitance whenthe second LNA 204 is off. By adding the additional capacitance of thegate capacitor 242, the input impedance during single mode more closelymatches the input impedance during split mode. Therefore, with both thegate switch 244 and the source switch 235 closed during single mode, theinput impedance will very nearly match the input impedance presentduring split mode (during which both the switches 244, 235 are opened).In single mode, the gate switch 244 and the source switch 235 areclosed. In split mode, the gate switch 244 and the source switch 235 areopen.

In addition, the front end amplifier 200 has at least a firstdegeneration switch 252 to disconnect a degeneration component, such asa first degeneration inductor 238, from the second LNA 204 during singlemode. In some embodiments, a second degeneration switch 254 is placedbetween the source of the first FET 210 and a second degenerationcomponent, such as a second degeneration inductor 236, to allow thedegeneration inductor 236 to be removed from the LNA 400. Accordingly,selection can be made as to which inductor 236, 238 to remove duringsingle mode. It will be clear to those of ordinary skill in the art thateither of the two degeneration switches 252, 254 can be provided aloneor the two switches 252, 254 may both be provided together.

A degeneration inductor 236, 238 is disconnected when the source switch235 is closed. Therefore, when the source switch 235 is open duringsplit mode, each LNA 202, 204 sees only the inductance of the onedegeneration inductor 236, 238 that is coupled to the respective sourceof the input FET 210, 214 associated with that LNA 202, 204. By openingone of the degeneration switches 252, 254 in single mode, the active LNA202, 214 operating in single mode has an inductive load between thesource and ground that is equal to the inductance of just one of thedegeneration inductors 236, 238, thus more closely matching theinductance presented during split mode. Providing a second degenerationswitch 254 provides flexibility as to which inductance to present at thesource of the active input FET 210, 214 no matter which output FET 208,212 is turned on during single mode.

In addition to the switches 235, 244, 252, 254, a drain switch 260 isprovided to enable the drain terminal of the input FET 210 to be coupledto the drain terminal of the input FET 214 during single mode. In anamplifier operating in single mode without a drain switch 260, the inputof one of the input FETs 210, 214 is essentially unused. Closing thedrain switch 260 places the two input FETs 210, 214 in parallel. Byplacing the two input FETs 210, 214 in parallel, the unused input FET210, 214 adds to the g_(m) (i.e., transconductance) of the input stageof the front end amplifier 200. Transconductance is the change in thedrain current divided by the small change in the gate to source voltagewith a constant drain to source voltage. In this case, the drain currentis the sum of the currents through each input FET 210, 214 (i.e., thecurrent through the FET 208, 212 that is conducting). Increases in theg_(m) of the amplifier result in an increase in the noise figure of theamplifier. The transition frequency is f_(T)≈g_(m)(2πC_(gs)). Therefore,because the value of C_(gs) is largely unchanged, the transitionfrequency (f_(T)) increases with an increase in g_(m).

When signals are routed to the LNA 1 output, the LNA 1 control signalapplied to the input 216 of the FET 208 provides bias to allow the FET208 to conduct. The LNA 2 control signal applied to the input of the FET212 is at ground potential to prevent the FET 212 from conducting. Byclosing the drain switch 260, the drain terminal of the input FET 214conducts in parallel with the input FET 210, shunting some of thecurrent that flows through the FET 208. It can be seen that essentiallytwice as much current will flow through the FET 208, since both theinput FETs 210, 214 are conducting (assuming that R_(on) of the drainswitch 260 is relatively low and the two LNAs 202, 204 are essentiallythe same). Doubling the overall current draw while maintaining the samecurrent through both input FETs 210, 214, doubles the effective inputdevice f_(T). However, the input FET 214 needs additional DC biascurrent to create g_(m), and so the amplifier 200 will consume morepower with both input FETs 210, 214 conducting. The bias of the inputFETs 210, 214 can be set to optimize f_(T) vs. power consumption.Accordingly, the amplifier 200 is flexible in its operation allowing theuser to determine how to balance the tradeoff between power consumptionand noise figure in single mode.

The drain switch 260 has a minimal impact on the operation of theamplifier 200 in split mode, since the drain switch 260 is open (i.e.,not conducting) during split mode. The effect of C_(off) (capacitance ofthe drain switch 260 when not conducting) on the forward signal isminimal, since the LNAs 202, 204 operate in common mode. However, If thedrain switch 260 is too large, it can negatively impact the isolation ofthe outputs LNA output 1 and LNA output 2 in split mode. Furthermore, alarge drain switch 260 increases the shunt capacitance to the substrateon which the components of the amplifier 200 are fabricated. Such anincrease in shunt capacitance can increase the noise contribution fromthe cascode. The use of SOI integrated circuitry mitigates this affectto some degree, since additional shunt capacitance added is typicallyrelatively small for silicon on insulator (SOI) integrated circuits.

In addition, the size of the drain switch 260 can have an impact on thenoise figure. If the drain switch 260 is too small, the DC voltage dropacross R_(on) of the drain switch 260 (i.e., the resistance through theswitch when the switch is closed) results in a decrease in the voltageVDS across the input FET 214. This results in a less efficient g_(m)which will have a negative impact on noise figure. Nonetheless, becauseR_(on) is presented after the g_(m) stage, the contribution of R_(on) tothe noise figure is not very significant.

FIG. 3 is a simplified schematic of another embodiment of an amplifier300 having source split LNAs 202, 204 and a drain switch 260. Theamplifier 300 includes a resistance that can selectively be placed inparallel across the inductor and capacitor in each of the output loadmatching circuits 220, 222. Within each output load matching circuits220, 222, a switch 302, 304 is coupled in series with a drain resistance306, 308 between the drain terminal of one of the output ports 224, 226and V_(DD). In single mode, the output impedance of the amplifier 300drops due to the input FETs 210, 214 being placed in parallel when thedrain switch 260 is closed. Opening the switches 302, 304 increases theresistance by removing the parallel path through each of the drainresistances 306, 308.

FIG. 4 is a graph illustrating the improvement in noise figure attainedby using the drain switch 260. A first curve 402 shows the noise figureassociated with each level of current I_(DD) drawn from the voltagesource V_(DD) during single mode with the drain switch open. In the caseof the embodiment shown in FIG. 3, the current I_(DD) is either: (1) thecurrent I_(DD1) flowing through the first LNA 202 when the LNA1 controlsignal is set to turn on the output FET 208 of LNA1 and any leakagecurrent I_(DD2) flowing through the second LNA 204; or (2) the currentI_(DD2) flowing through the second LNA 204 when the LNA2 control signalis set to turn on the output FET 212 of LNA2 and any leakage currentI_(DD1) flowing through the first LNA 202. A second curve 404 shows thenoise figure associated with each level of current I_(DD) drawn from thevoltage source V_(DD) in single mode with the drain switch closed. Itcan be seen that at a point 406 on the curve 402 at which the currentI_(DD) is equal to 6 mA, the noise figure with the drain switch open isapproximately 1.55 dB. In contrast, at the point 408 the noise figurewith the drain switch 260 closed is approximately 1.46 dB. Theimprovement in noise figure increases with greater I_(DD). With thecurrent I_(DD) is 16 mA, the noise figure with the drain switch 260 openis shown at point 410 to be approximately 1.53 dB. In contrast, thenoise figure with the drain switch 260 closed shown at point 412 isapproximately 1.23 dB. It should be noted that these curves were plottedfrom an amplifier 300 having nominal values for the components and ismeant merely to give a relative scale to the amount of improvementattainable. Amplifiers with components having particular values andcharacteristics may vary from these numbers.

FIG. 5 is a graph and chart showing the relationship between the currentI_(DD) and noise figure as a function of input frequency. Four curves502, 504, 506, 508 are shown plotted over a frequency range from 2 GHzto 3 GHz. The first curve 502 represents a current I_(DD) equal to 9.1mA with the drain switch open over the frequencies from 2 GHz to 3 GHz.The second curve 504 illustrates the noise figure over the frequencyrange for a current I_(DD) of 3.9 mA with the drain switch 260 closed.The third curve 506 illustrates the noise figure over the frequencyrange for a current I_(DD) of 7.59 mA with the drain switch 260 closed.The fourth curve 508 illustrates the noise figure over the frequencyrange for a current I_(DD) of 11.0 mA with the drain switch 260 closed.It can be seen that with the drain switch 260 closed, nearly the samenoise figure can be attained with only 7.5 mA of I_(DD) compared to 9.1mA with the drain switch 260 open.

In accordance with some embodiments of the disclosed method andapparatus, the switches 235, 244, 252, 254, 260, 302, 304 can bemanufactured in accordance with techniques provided in U.S. Pat. No.6,804,502 (the “502 patent”), which is incorporated by reference herein,and disclosed in other related patents. Additional improvements in theperformance of one or more of the switches 235, 244, 252, 254, 260, 302,304 can be attained by implementing the techniques provided in U.S. Pat.No. 7,910,993 (the “993 patent”), which is incorporated by referenceherein, and disclosed in other related patents. Use of such highperformance switches reduces the non-linearity of the switches and thusthe adverse effects of such switches on the performance of the receiver.However, in many implementations, it may be possible to use switchesthat have performance characteristics (i.e., linearity, return loss,switching speed, ease of integration, etc.) that are not as good as thecharacteristics of switches made in accordance with the techniquesdisclosed in the '502 and '993 patents. Accordingly, each or some of theswitches disclosed above can be implemented using any combination of oneor more transistors, including FETs, bipolar junction transistors(BJTs), or any other semiconductor switch. Alternatively, the switchescan be implemented by electromechanical or MEMS(Micro-Electro-Mechanical Systems) technologies.

In addition, it will be understood by those skilled in the art that eachof the switches 235, 244, 252, 254, 260, 302, 304 can be controlled by acontrol signal generated by the LNA controller 217 or other suchcontroller to select the state of each switch as a function of the modeof operation of the amplifier (i.e., whether in single mode or splitmode). Such control signals and inputs to the switches are not shown forthe sake of simplicity in the figures, but are well within theunderstanding of those skilled in the art.

Methods

FIG. 6 is an illustration of a method in accordance with one embodimentfor amplifying a signal (e.g., a CA signal) using more than oneamplifier. The signal is applied to the input of the amplifiers [STEP601]. In some embodiments, the signal includes a first and a secondnon-adjacent channel. The first and second channels are considered to benon-adjacent if there is at least a narrow frequency range between thedefined end of the frequency range of the first channel and the definedbeginning of the frequency range of the second channel. Typically, atleast a third channel is defined within the frequency range between theend of the first and beginning of the second channel. The frequencyrange of a channel is typically defined by industry standards, but insome cases may be defined by the 3 dB frequency range of filterscommonly used to receive signals transmitted over the channel.

The method further includes selecting between a single mode or a splitmode [STEP 603]. In one embodiment, the selection between single modeand split mode is made by turning on an output FET 208 in a first LNA202 and turning off a second output FET 121 within a second LNA 204 toselect single mode [STEP 605]. In one such embodiment, the first outputFET 208 is turned on by applying an LNA control signal to a firstcontrol input terminal 216 coupled to the gate of the output FET, suchas the FET 208 shown in FIGS. 2-5. The second output FET 212 is turnedoff by applying an LNA control signal to a second control input terminal218. Similarly, the selection of split mode is made by applying LNAcontrol signals to the control terminals 216, 218 to turn both outputFETs 208, 212 on [STEP 607].

The method further includes coupling the source of an input FET of thefirst LNA 202, such as FET 210 and the source of an input FET of thesecond LNA 204, such as the FET 212, during single mode [STEP 609] anddecoupling the two sources during split mode [STEP 611]. In one suchembodiment, a source switch 235 is closed in single mode and opened insplit mode. When closed, the source switch 235 couples the two sourcesof the input FETs 210, 212. Furthermore, the drain switch 260 is closedin single mode [STEP 613] and opened during split mode [STEP 615].

FIG. 7 is an illustration of another embodiment of a method thatincludes removing parallel drain resistances 306, 308 during single mode[STEP 713]. In one embodiment, the drain resistances are removed byopening switches 302, 304. The resistances 306, 308 are added duringsplit mode [STEP 715] by closing the switches 302, 304. One suchembodiment further includes selecting the resistance value of the drainresistances such that the output impedance is essentially the sameduring single mode and split mode.

Fabrication Technologies and Options

As should be readily apparent to one of ordinary skill in the art,various embodiments of the claimed invention can be implemented to meeta wide variety of specifications. Unless otherwise noted above,selection of suitable component values is a matter of design choice andvarious embodiments of the claimed invention may be implemented in anysuitable IC technology (including but not limited to MOSFET and IGFETstructures), or in hybrid or discrete circuit forms. Integrated circuitembodiments may be fabricated using any suitable substrates andprocesses, including but not limited to standard bulk silicon,silicon-on-insulator (SOI), silicon-on-sapphire (SOS), GaN HEMT, GaAspHEMT, and MESFET technologies. However, in some cases, the inventiveconcepts claimed may be particularly useful with an SOI-basedfabrication process (including SOS), and with fabrication processeshaving similar characteristics.

A number of embodiments of the claimed invention have been described. Itis to be understood that various modifications may be made withoutdeparting from the spirit and scope of the invention. For example, someof the steps described above may be order independent, and thus can beperformed in an order different from that described. Further, some ofthe steps described above may be optional. Various activities describedwith respect to the methods identified above can be executed inrepetitive, serial, or parallel fashion. It is to be understood that theforegoing description is intended to illustrate and not to limit thescope of the claimed invention, which is defined by the scope of thefollowing claims, and that other embodiments are within the scope of theclaims.

What is claimed is:
 1. An amplifier including: (a) a first low noiseamplifier, LNA, including an input transistor and an output transistor,each transistor having a control input, a source, and a drain, whereinthe drain of the input transistor is series-coupled to the source of theoutput transistor at a common connection point; (b) a second LNAincluding an input transistor and an output transistor, each transistorhaving a control input, a source, and a drain, wherein the drain of theinput transistor is series-coupled to the source of the outputtransistor at a common connection point; and (c) a drain switchconnecting the common connection points of the first and second LNAs ina single mode of operation in which one of the output transistors isoff, and disconnecting the common connection points of the first andsecond LNAs in a split mode of operation.
 2. The amplifier of claim 1,further including a source switch connecting the sources of the firstand second LNAs in the single mode of operation, and disconnecting thesources of the first and second LNAs in the split mode of operation. 3.The amplifier of claim 1, further including a switchable gatecapacitance module coupled between the source and control input of theinput transistor of the first LNA and configured to couple a capacitancebetween the source and control input of the input transistor of thefirst LNA in the single mode of operation, and to disconnect thecapacitance in the split mode of operation.
 4. The amplifier of claim 3,wherein the capacitance of the switchable gate capacitance module isselected such that an input impedance of the first LNA is essentiallythe same in the single mode of operation and the split mode ofoperation.
 5. The amplifier of claim 1, further including a firstswitchable degeneration component coupled to the source of the inputtransistor of the first LNA, and a second switchable degenerationcomponent coupled to the source of the input transistor of the secondLNA, wherein the first and second switchable degeneration components areconfigured to couple a degeneration inductance to the respective sourcesof the input transistors of the first and second LNAs in the split modeof operation, and to disconnect the degeneration inductance of at leastone of the first and second switchable degeneration components in thesingle mode of operation.
 6. The amplifier of claim 1, further includinga first switchable load impedance component coupled to the drain of theoutput transistor of the first LNA, and a second switchable loadimpedance component coupled to the drain of the output transistor of thesecond LNA, wherein the first and second switchable load impedancecomponents are configured to couple a load impedance to the respectivedrains of the output transistors of the first and second LNAs in thesplit mode of operation, and to disconnect the load impedance fromrespective drains of the output transistors of the first and second LNAsin the single mode of operation.
 7. The amplifier of claim 1, wherein inthe single mode of operation in which one of the output transistors isoff, the other of the output transistors is on and the current throughthe output transistor that is on is the sum of the currents through theinput transistors of the first and second LNAs.
 8. The amplifier ofclaim 1, further including: (a) a source switch connecting the sourcesof the first and second LNAs in the single mode of operation, anddisconnecting the sources of the first and second LNAs in the split modeof operation; and (b) a switchable gate capacitance module coupledbetween the source and control input of the input transistor of thefirst LNA and configured to couple a capacitance between the source andcontrol input of the input transistor of the first LNA in the singlemode of operation, and to disconnect the capacitance in the split modeof operation.
 9. The amplifier of claim 1, further including: (a) asource switch connecting the sources of the first and second LNAs in thesingle mode of operation, and disconnecting the sources of the first andsecond LNAs in the split mode of operation; (b) a switchable gatecapacitance module coupled between the source and control input of theinput transistor of the first LNA and configured to couple a capacitancebetween the source and control input of the input transistor of thefirst LNA in the single mode of operation, and to disconnect thecapacitance in the split mode of operation; and (c) a first switchableload impedance component coupled to the drain of the output transistorof the first LNA, and a second switchable load impedance componentcoupled to the drain of the output transistor of the second LNA, whereinthe first and second switchable load impedance components are configuredto couple a load impedance to the respective drains of the outputtransistors of the first and second LNAs in the split mode of operation,and to disconnect the load impedance from respective drains of theoutput transistors of the first and second LNAs in the single mode ofoperation.
 10. The amplifier of claim 1, further including: (a) a sourceswitch connecting the sources of the first and second LNAs in the singlemode of operation, and disconnecting the sources of the first and secondLNAs in the split mode of operation; (b) a switchable gate capacitancemodule coupled between the source and control input of the inputtransistor of the first LNA and configured to couple a capacitancebetween the source and control input of the input transistor of thefirst LNA in the single mode of operation, and to disconnect thecapacitance in the split mode of operation; and (c) a first switchabledegeneration component coupled to the source of the input transistor ofthe first LNA, and a second switchable degeneration component coupled tothe source of the input transistor of the second LNA, wherein the firstand second switchable degeneration components are configured to couple adegeneration inductance to the respective sources of the inputtransistors of the first and second LNAs in the split mode of operation,and to disconnect the degeneration inductance of at least one of thefirst and second switchable degeneration components in the single modeof operation.
 11. The amplifier of claim 1, further including: (a) asource switch connecting the sources of the first and second LNAs in thesingle mode of operation, and disconnecting the sources of the first andsecond LNAs in the split mode of operation; (b) a switchable gatecapacitance module coupled between the source and control input of theinput transistor of the first LNA and configured to couple a capacitancebetween the source and control input of the input transistor of thefirst LNA in the single mode of operation, and to disconnect thecapacitance in the split mode of operation; (c) a first switchabledegeneration component coupled to the source of the input transistor ofthe first LNA, and a second switchable degeneration component coupled tothe source of the input transistor of the second LNA, wherein the firstand second switchable degeneration components are configured to couple adegeneration inductance to the respective sources of the inputtransistors of the first and second LNAs in the split mode of operation,and to disconnect the degeneration inductance of at least one of thefirst and second switchable degeneration components in the single modeof operation; and (d) a first switchable load impedance componentcoupled to the drain of the output transistor of the first LNA, and asecond switchable load impedance component coupled to the drain of theoutput transistor of the second LNA, wherein the first and secondswitchable load impedance components are configured to couple a loadimpedance to the respective drains of the output transistors of thefirst and second LNAs in the split mode of operation, and to disconnectthe load impedance from respective drains of the output transistors ofthe first and second LNAs in the single mode of operation.
 12. Theamplifier of claim 1, further including: (a) a source switch connectingthe sources of the first and second LNAs in the single mode ofoperation, and disconnecting the sources of the first and second LNAs inthe split mode of operation; (b) a first switchable degenerationcomponent coupled to the source of the input transistor of the firstLNA, and a second switchable degeneration component coupled to thesource of the input transistor of the second LNA, wherein the first andsecond switchable degeneration components are configured to couple adegeneration inductance to the respective sources of the inputtransistors of the first and second LNAs in the split mode of operation,and to disconnect the degeneration inductance of at least one of thefirst and second switchable degeneration components in the single modeof operation; and (c) a first switchable load impedance componentcoupled to the drain of the output transistor of the first LNA, and asecond switchable load impedance component coupled to the drain of theoutput transistor of the second LNA, wherein the first and secondswitchable load impedance components are configured to couple a loadimpedance to the respective drains of the output transistors of thefirst and second LNAs in the split mode of operation, and to disconnectthe load impedance from respective drains of the output transistors ofthe first and second LNAs in the single mode of operation.
 13. Theamplifier of claim 1, further including: (a) a source switch connectingthe sources of the first and second LNAs in the single mode ofoperation, and disconnecting the sources of the first and second LNAs inthe split mode of operation; and (b) a first switchable degenerationcomponent coupled to the source of the input transistor of the firstLNA, and a second switchable degeneration component coupled to thesource of the input transistor of the second LNA, wherein the first andsecond switchable degeneration components are configured to couple adegeneration inductance to the respective sources of the inputtransistors of the first and second LNAs in the split mode of operation,and to disconnect the degeneration inductance of at least one of thefirst and second switchable degeneration components in the single modeof operation.
 14. The amplifier of claim 1, further including: (a) asource switch connecting the sources of the first and second LNAs in thesingle mode of operation, and disconnecting the sources of the first andsecond LNAs in the split mode of operation; and (b) a first switchableload impedance component coupled to the drain of the output transistorof the first LNA, and a second switchable load impedance componentcoupled to the drain of the output transistor of the second LNA, whereinthe first and second switchable load impedance components are configuredto couple a load impedance to the respective drains of the outputtransistors of the first and second LNAs in the split mode of operation,and to disconnect the load impedance from respective drains of theoutput transistors of the first and second LNAs in the single mode ofoperation.
 15. The amplifier of claim 1, further including: (a) aswitchable gate capacitance module coupled between the source andcontrol input of the input transistor of the first LNA and configured tocouple a capacitance between the source and control input of the inputtransistor of the first LNA in the single mode of operation, and todisconnect the capacitance in the split mode of operation; (b) a firstswitchable degeneration component coupled to the source of the inputtransistor of the first LNA, and a second switchable degenerationcomponent coupled to the source of the input transistor of the secondLNA, wherein the first and second switchable degeneration components areconfigured to couple a degeneration inductance to the respective sourcesof the input transistors of the first and second LNAs in the split modeof operation, and to disconnect the degeneration inductance of at leastone of the first and second switchable degeneration components in thesingle mode of operation; and (c) a first switchable load impedancecomponent coupled to the drain of the output transistor of the firstLNA, and a second switchable load impedance component coupled to thedrain of the output transistor of the second LNA, wherein the first andsecond switchable load impedance components are configured to couple aload impedance to the respective drains of the output transistors of thefirst and second LNAs in the split mode of operation, and to disconnectthe load impedance from respective drains of the output transistors ofthe first and second LNAs in the single mode of operation.
 16. Theamplifier of claim 1, further including: (a) a switchable gatecapacitance module coupled between the source and control input of theinput transistor of the first LNA and configured to couple a capacitancebetween the source and control input of the input transistor of thefirst LNA in the single mode of operation, and to disconnect thecapacitance in the split mode of operation; and (b) a first switchabledegeneration component coupled to the source of the input transistor ofthe first LNA, and a second switchable degeneration component coupled tothe source of the input transistor of the second LNA, wherein the firstand second switchable degeneration components are configured to couple adegeneration inductance to the respective sources of the inputtransistors of the first and second LNAs in the split mode of operation,and to disconnect the degeneration inductance of at least one of thefirst and second switchable degeneration components in the single modeof operation.
 17. The amplifier of claim 1, further including: (a) aswitchable gate capacitance module coupled between the source andcontrol input of the input transistor of the first LNA and configured tocouple a capacitance between the source and control input of the inputtransistor of the first LNA in the single mode of operation, and todisconnect the capacitance in the split mode of operation; and (b) afirst switchable load impedance component coupled to the drain of theoutput transistor of the first LNA, and a second switchable loadimpedance component coupled to the drain of the output transistor of thesecond LNA, wherein the first and second switchable load impedancecomponents are configured to couple a load impedance to the respectivedrains of the output transistors of the first and second LNAs in thesplit mode of operation, and to disconnect the load impedance fromrespective drains of the output transistors of the first and second LNAsin the single mode of operation.
 18. The amplifier of claim 1, furtherincluding: (a) a first switchable degeneration component coupled to thesource of the input transistor of the first LNA, and a second switchabledegeneration component coupled to the source of the input transistor ofthe second LNA, wherein the first and second switchable degenerationcomponents are configured to couple a degeneration inductance to therespective sources of the input transistors of the first and second LNAsin the split mode of operation, and to disconnect the degenerationinductance of at least one of the first and second switchabledegeneration components in the single mode of operation; and (b) a firstswitchable load impedance component coupled to the drain of the outputtransistor of the first LNA, and a second switchable load impedancecomponent coupled to the drain of the output transistor of the secondLNA, wherein the first and second switchable load impedance componentsare configured to couple a load impedance to the respective drains ofthe output transistors of the first and second LNAs in the split mode ofoperation, and to disconnect the load impedance from respective drainsof the output transistors of the first and second LNAs in the singlemode of operation.
 19. An amplifier including: (a) a first low noiseamplifier, LNA, including an input transistor having a drainseries-coupled to a source of an output transistor at a commonconnection point; (b) a second LNA including an input transistor havinga drain series-coupled to a source of an output transistor at a commonconnection point; and (c) a drain switch connecting the commonconnection points of the first and second LNAs during a single mode ofoperation in which one of the output transistors is off such that thecurrent through one of the output transistors is the sum of the currentsthrough the input transistors, and disconnecting the common connectionpoints of the first and second LNAs during a split mode of operation.20. An amplifier including: (a) a plurality of low noise amplifiers,LNA, each including an input transistor and an output transistor, theinput transistor having a drain series-coupled to a source of the outputtransistor at a common connection point; at least one drain switchconnecting the common connection points of at least two of the LNAs suchthat the current through one of the output transistors is the sum of thecurrents through the input transistors in a single mode of operation inwhich all but one of the output transistors of the plurality of LNAs isoff, and disconnecting the common connection points in a split mode ofoperation in which the output transistors of at least two of pluralityof LNAs are on.